This invention relates generally to electronic circuits and specifically to an analog pre-processor that can be used with a digital imaging device.
Digital cameras are fast replacing film based cameras as a medium to capture and store images. In a digital camera, a light sensor converts the incident light into voltage. The analog front end (AFE) first processes the signal to improve dynamic range and subsequently digitizes this voltage. The digital output is then processed further and stored so it can be viewed in various forms.
The main thrust in digital camera research is to improve picture quality and reduce power. The analog front end is one of the key blocks which determines the overall accuracy and power of the camera system. The AFE can be conceptually divided into two parts, the analog signal processing section (referred to here as the analog pre-processor or APP) followed by a digitizer (e.g., an analog-to-digital converter or ADC).
For hand-held digital cameras and camcorders, a significant performance criteria is power, for a given accuracy. Camera vendors are looking for very low power twelve-bit analog front ends. For example, one goal is to produce sub-80 mW AFEs. This goal represents a power reduction of about one third compared to commercially available devices.
In one aspect, the present invention provides an analog pre-processor that achieves both low power and high accuracy. For example, the preferred embodiment provides a single operational amplifier (op amp) based architecture that minimizes analog power as well as provides optimum noise and linearity. This device is followed by a very low power twelve-bit analog to digital converter to complete the front end.
The present invention is useful in a number of applications including in a digital imaging device such as a digital camera or camcorder. This application might include a light sensor such as a charge coupled device (CCD) or a CMOS integrated sensor (CIS). Electrical signals corresponding to received light is provided to an analog pre-processor. In the preferred embodiment, the analog pre-processor includes a single amplifier. After the pre-processing, the signal can be converted to a digital signal and further processed prior to being displayed and/or stored.
In a more specific embodiment, the analog pre-processor is an analog circuit that includes an amplifier with a positive input node, a negative input node, a positive output node and a negative output node. A first variable capacitor is coupled between the positive input node and an analog signal node. A second variable capacitor is coupled between the negative input node and a reference voltage node. In addition, a third variable capacitor is coupled between the positive input node and the negative output node and a fourth variable capacitor is coupled between the negative input node and the positive output node. A first switch is coupled between the third variable capacitor and the negative output node and a second switch is coupled between the fourth variable capacitor and the positive output node.
The preferred embodiment includes a number of advantages over alternative solutions. For example, the pre-processor utilizes very little power. This low power is accomplished while maintaining good PGA (programmable gain amplifier) linearity. In addition, the noise is comparable or better than other twelve-bit parts.